The present invention relates to a hydraulic control system for use in mineral mining installations and to mineral mining installations employing such a system.
In general, the control system is intended for controlling the operation of a series of double-acting shifting rams.
In various forms of mining apparatus it is known to use horizontal shifting rams. For example, such rams are used to horizontally advance a longwall conveyor in relation to roof support chocks or frames. Hitherto control systems have been used to supply hydraulic pressure fluid to the rams so that when a ram is to be extended one of its working chambers is supplied with pressure fluid and the other of its working chambers is connected to a fluid return path. Conversely, when the ram is to be retracted the situation is reversed with the other working chamber now being supplied with fluid.
It is also usual for the piston surface area exposed to the fluid in the first chamber, supplied to extend the ram, to be greater than that exposed to the fluid in the other chamber, supplied to retract the ram. The ram may have its cylinder secured to a vertically extending roof support chock or frame and its piston rod secured to the conveyor, for example, so that with the chocks or frame set or braced vertically between the roof and floor the ram can be horizontally extended to advance the conveyor and with the chock or frame released the ram can be retracted to draw up the frame towards the conveyor. During the mineral winning process it is desirable for the ram to urge the conveyor and hence the winning machine against the mineral face or seam but hitherto it has not been possible to effect any suitable controlled adjustment of the force applied by the rams to cope with the changing conditions. Although it is known from German Pat. No. 1,058,003 to supply compressed air to the other working chamber of a ram, used for retraction, while the first working chamber, used for extension is supplied with pressure fluid at a greater pressure, this counter-force is constant and serves merely to resiliently damp the piston of the ram so there is no control of the force of the ram.
A general object of this invention is to provide an improved form of control system for use with mining apparatus of the aforementioned kind.